1. Field of the Invention
The present invention relates to an optical receiver, an optical receiving apparatus using the optical receiver and an optical receiving method thereof, more particularly relates to techniques to control the power level of an optical signal received at the optical receiver, the optical receiving apparatus and the optical receiving method used for wavelength division multiplexing (WDM) optical transmission systems.
2. Description of the Related Art
An optical receiving apparatus used for conventional wavelength division multiplexing (WDM) optical transmission systems is disclosed in reference document 1 (Japanese Patent Application Laid-open Publication No. 2003-023399, FIG. 29). FIG. 6 shows the WDM optical transmission system. A wavelength division multiplexed (WDM) optical signal is demultiplexed by a demultiplexer (DEMUX) 125 and the demultiplexed optical signals are input to each optical receiver. Each optical receiver has an optical amplifier 127 and an automatic level control unit (ALC) 128. The ALC 128 controls the output-level of the optical amplifier 127 by providing feedback thereto. Therefore, each optical signal has been received by each receiver 126 at a constant power level. Consequently, even if wavelength fluctuations happen to an optical signal input to certain receiver 126, input-level to the receiver 126 is maintained constant because the output-level from the optical amplifier 127 is maintained constant.
However, a light receiving element in the receiver 126 generally has a property that quantum efficiency is different for each wavelength, so-called “wavelength dependence of quantum efficiency”. The wavelength dependence of quantum efficiency that a typical light receiving element has is shown in FIG. 7 (refer to “Hikari Jyouhou Nettowaku (Optical Information Network)” edited by Kazuro Kikuchi, Ohmsha, October, 2002, p. 169). The horizontal axis indicates wavelengths and the vertical axis indicates quantum efficiency. Quantum efficiency of InGaAs/InP, which is mainly used in WDM optical transmission systems at present, tends to deteriorate sharply at around 1.6 μm. Therefore, when a wavelength of light input to the light receiving element varies, electrical output-level from the light receiving element varies. As a result, in the reference document 1, when the wavelength varies, the electrical output-level from the light receiving element varies even if an optical input-level to the light receiving element is maintained constant. Until now, the fluctuations that happen in electrical output-level have been absorbed by an electrical circuit arranged at a stage after the light receiving element.
As for optical fiber communications in recent years, a speedup is required with enlarging capacity of information transmission, and therefore research and development of a 40 Gbps system has been carried out. The high speed optical transmission system of 40 Gbps or more places a severe constraint on the dynamic range of an electrical circuit arranged at a stage after the light receiving element. For example, in a clock-regeneration and discrimination circuit as an electrical circuit, the minimum distinguishable amplitude of the 40 Gbps system is about one order of magnitude larger than that of a 10 Gbps system. For that reason, the allowable amount of output amplitude fluctuations in the 40 Gbps system is about ten times smaller than that in the 10 Gbps system. Consequently, the 40 Gbps system becomes about ten times weaker than the 10 Gbps system in amplitude fluctuations of an electrical signal generated by wavelength fluctuations of an optical signal. As a result, the 40 Gbps system requires accuracy as high as about ten times the 10 Gbps system in amplitude adjustments which are performed according to the wavelength fluctuations.
As mentioned above, with the increase of transmission speed in recent years, fluctuations in the electrical output-level due to the wavelength dependence of quantum efficiency cannot be disregarded, and thereby the fluctuations have become a factor that makes transmission characteristics worse.